Heaters of this type are, e.g., space heaters for passenger cars, trucks or buses, which operate independently of the drive engine of the motor vehicle. The liquid fuel (gasoline, diesel fuel) is burned in a burner while combustion air is supplied. The combustion chamber is typically surrounded by a heat exchanger through which air or water flows.
For safety reasons, regulations normally require that the fuel supply be stopped within a certain period of time in the case of interruption of the flame in order to prevent the risk of explosion or the like, because interruption of the flame during the operation of a heater usually means a malfunction that must be eliminated by all means.
Interruption of the flame can be recognized in many different ways: for example, the use of an optical sensor is common and known. However, this solution may be problematic because of the difficulty of arranging the optical sensor in the appropriate place.
Another possibility for flame monitoring is to measure the temperatures in the combustion space or at the heat exchanger or at the combustion chamber to infer interruption of the flame from a drop in temperature. However, a drop in temperature outside the combustion chamber or at the heat exchanger that can be evaluated by the flame monitoring device takes place only if there is no stagnant air in the combustion chamber or at the heat exchanger. For example, if the flame has been interrupted and the combustion air blower is also not in operation, the heated air will stagnate in the combustion chamber, so that the temperature on the combustion chamber drops only slowly. The drop in temperature is thus recognized too late, long after the expiration of the required period of time within which the fuel supply has to be stopped.
Consequently, delivery of combustion air into the combustion chamber or blowing of heating air past the heat exchanger must be ensured even after interruption of the flame. It is only under these conditions that the temperature on the combustion chamber or in the heat exchanger will drop rapidly enough after interruption of the flame to permit the interruption of the flame to be recognized.
Measuring the power consumption of the motor to monitor the electric motor driving the blower has been common practice hitherto. If the power consumption is within a certain nominal range, this is evaluated as implying proper operation of the blower motor. Monitoring the power consumption by a reed relay operated with electric current in conjunction with a fuse in the motor circuit has also been common practice.
A safety circuit for engine-independent heaters for motor vehicles, which affects the operation of the heating blower, is disclosed in DE (West German) Patent Specification No. 37,38,739 A1. To prevent overheating in the case of malfunction of the heating blower, a coil, which is associated with a Hall generator, is included in the circuit of the heating air blower. Under normal conditions, the Hall generator generates a signal characterizing normal operation, because it senses the magnetic field generated by the coil through which the current flows. With the motor of the heating air blower stopped, no current flows, and the coil does not generate any magnetic field, so that the Hall generator sends an appropriate malfunction signal to the control device, which will thereupon interrupt the fuel supply.